When thinking of renovating the windows of old buildings, the building owner has a lot of decisions to make. These are to keep the window but make it more energy efficient by adding an extra pane or to completely change the whole window. At the same time, the joint between the window frame and wall makes a thermal bridge which depends on how much insulation has been placed in the cavities after installation. Upon the decision of keeping the window, the status of this joint will be unchanged. This thesis deals with finding out the best economical solution for a company that has rental apartments in Gävle in Sweden, Gavlegårdarna AB. To calculate the thermal bridges, which are weak areas of the building envelope in which they significantly increase the energy load of houses, a CDF program called Fluent is used to quantify the heat loss at the joints. Measurements have been performed to validate the CFD model. To simulate the energy savings in the building, the building energy simulation program IDA-ICE is used. Finally, Life Cycle Costing calculations are made to assess the best long term economical option. It is concluded that the most reasonable solution is to add an extra glass in the existing window, but it is not the most ecofriendly. A more ecological solution is to add the extra glass and to perform enhanced insulation at the joints around the window frames and walls, or to replace the old window with a new low energy window – however, these are not profitable solutions.